Electro-optical display element

ABSTRACT

A suitable choice of the liquid crystal parameters K 3  /K 1  and ΔE/E 1  makes it possible in electrooptical display elements to obtain both a minimum dependence on the angle of observation and, simultaneously, a steep electro-optical characteristic when operated in the first transmission minimum.

This application is a continuation of application Ser. No. 06/774,680,filed Sept. 11, 1985, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to an electro-optical display element with a verysmall dependence of the contrast on the angle of observation and with aparticularly steep electro-optical characteristic curve.

The properties of nematic or nematic-cholesteric liquid-crystallinematerials are utilized for liquid-crystal display elements in order toeffect significant changes in their optical properties, such as lighttransmission, light scattering, birefringence, reflectance or color,under the influence of electric fields. The functioning of displayelements of this type is based here, for example, on the phenomena ofdynamic scattering, the deformation of aligned phases or theSchadt-Helfrich effect in the twisted cell.

Among these liquid-crystal display elements, twisted nematic cells (TNcells) have gained particular importance recently, because they can beoperated with relatively low control voltages which can be readily madeavailable even by small batteries. Moreover, these display elements havehitherto been the best which can be constructed as matrix displayelements which allow the presentation of a high information densitywithout an intolerably large numer of control lines, lead-ins andlead-outs.

When used in practice, however, the TN cell, in particular in the formof matrix display elements, still raises difficulties. These includeabove all the more or less pronounced dependence of the contrast on theangle of observation and the restricted multiplexing capacity.

Proposed solutions for both problems are already to be found in theliterature, and some of these have been put into practice. Thus, forexample, the dependence of the contrast on the angle of observation in aTN cell can be significantly improved by suitable choice of therefractive index anisotropy Δn and of the cell thickness d [L. Pohl, G.Weber, R. Eidenschink, G. Baur and W. Fehrenbach, Appl. Phys. Lett. 38(1981) 497].

Particularly in the so-called first transmission minimum according toGooch and Tarry [C. H. Gooch and H. A. Tarry, J. Phys. D8 (1975) 1575],the dependence of the contrast on the angle of observation is at aminimum. This requires the condition ##EQU1## to be met (λ=wavelength ofthe light used).

The multiplexing capacity of a TN cell is determined by the liquidcrystal parameters K₁,K₂,K₃ (elastic constants for spreading, twistingand bending), by the dielectric constants ε.sub.⊥ and ε.sub.∥, and bythe refractive indices n_(o) and n_(e), and also by cell parameters,such as, for example, the twist angle and tilt angle at the surface ofthe substrate. The influence of these material parameters and cellparameters has already been investigated by several authors [D. W.Berreman J. Appl. Phys. 46 (1975) 3746; F. Gharadjedaghi and J. Robert,Rev. Phys. Appl. 11 (1976) 467; G. Baur, in "The Physics and Chemistryof Liquid Crystal Devices" (edited by G. J. Sprokel) Plenum, N.Y.,(1981) pages 61 et seq.; G. Baur, Mol. Crystl. Liq. Cryst. 63 (1981) 45;C. Z. van Doorn, C. J. Gerritsma and J. J. M. J. de Klerk, in "ThePhysics and Chemistry of Liquid Crystal Devices" (edited by G. J.Sprokel) Plenum, N.Y., (1980), pages 95 et seq.; F. J. Kahn and H.Birecki, in "The Physics and Chemistry of Liquid Crystal Devices"(edited by G. J. Sprokel) Plenum, N.Y., (1980a), (1980b), pages 125 etseq].

According to an approximation equation, the multiplexing capacity isestimated by ##EQU2## [M. Schadt and P. R. Gerber, ZeitschriftNaturforsch. 37a (1982) 165].

The smaller p, the higher the multiplexing capacity. According to thisestimation, only the K₃ /K₁ ratio and Δn.d affect the steepness of theelectro-optical characteristic curve and hence the multiplexingcapacity. The K₂ /K₁ and (ε.sub.∥ -ε.sub.⊥)/ε.sub.⊥ =Δε/ε.sub.⊥ ratiosare disregarded.

Experimental investigations of TN cells with the liquid crystalmaterials hitherto predominantly used have shown that the steepness ofthe electro-optical characteristic in the second transmission minimum isbetter than that in the first minimum, whereas the dependence on theangle of observation in the first minimum is better than that in thesecond transmission minimum.

By means of the proposed solutions hitherto known, the most importantdifficulties arising in the construction and use of TN cells canadmittedly be solved individually, but not simultaneously.

The disclosures of all of the references cited above are incorporated byreference herein.

SUMMARY OF THE INVENTION

It is an object of the invention to provide electro-optical displayelements, in which both a minimum dependence on the angle of observationand, simultaneously, also the steepest possible electro-opticalcharacteristic are ensured by operating the display elements in thefirst transmission minimum according to Gooch and Tarry.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

It has now been found that, with a suitable choice of the liquid crystalparameters, it is possible, surprisingly, to obtain both minimumdependence on the angle of observation and, simultaneously, a steepelectro-optical characteristic curve by operating the display elementsin the first transmission minimum.

A subject of the invention, therefore, is an electro-optical displayelement with a liquid-crystalline phase comprising at least twocomponents, and wherein the electro-optical characteristic in the firsttransmission minimum is at least as steep as that in the secondtransmission minimum.

The invention also relates to a method of simultaneously minimizing thedependence of contrast on the angle of observation of an electro-opticaldisplay element based on a liquid crystal phase and of maximizing itsmultiplexing capacity by maximizing its steepness of its characteristiccurve, comprising operating the element in the first Gooch and Tarrytransmission minimum, and selecting a phase having a K₃ /K₁ ratio and aΔε/ε.sub.⊥ ratio consistent with the attainment of a characteristiccurve in the first transmission minimum which is steeper than that inthe second such transmission minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will become more fully appreciated as the same becomes betterunderstood when considered in conjunction with the accompanyingdrawings, in which like reference characters designate the same orsimilar parts throughout the several views, and wherein:

FIG. 1 contains a diagram of K₃ /K₁ values vs. Δε/ε.sub.⊥ values whichis useful in implementing this invention

DETAILED DISCUSSION

The construction of the liquid crystal display element according to theinvention from polarizers, electrode base plates and electrodes withsuch a surface treatment that the preferential orientation of theparticular adjoining liquid crystal molecules is usually twisted by 90°from one electrode to the other, corresponds to the type of constructionconventional for such display elements. The concept of the conventionaltype of construction is here taken in a very wide sense and alsocomprises all the variations and modifications, known from theliterature, of the twisted nematic cell, and in particular also matrixdisplay elements and the display elements according to GermanOffenlegungsschrift 2,748,738 which additionally contain magnets. Anessential difference between the display elements according to theinvention and those hitherto conventional and based on the twistednematic cell is, however, the choice of the liquid crystal parameters inthe liquid crystal layer. Whereas, for example, the K₃ /K₁ ratio in theconventional liquid-crystalline phases for display elements of this typeis >0.8 and the Δε/ε.sub.⊥ ratio is normally between 1.2 and 1.7, theK.sub. 3 /K₁ ratio in the display elements according to the invention is≦0.8.

In the display elements according to the invention, liquid-crystallinephases are used in which the liquid crystal parameters K₃ /K₁ andΔε/ε.sub.⊥ are chosen such that both a minimum dependence on the angleof observation and, simultaneously, also the steepest possibleelectro-optical characteristic are ensured by operating the displayelements in the first transmission minimum.

FIG. 1 is a diagrammatic drawing which illustrates those pairs of valuesof the parameters K₃ /K₁ and Δε/ε.sub.⊥ which can be chosen (below thedashed area) in order to ensure that the electro-optical characteristicin the first transmission minimum is steeper than that in the second.Above the dashed area the electro-optical characteristic in the secondtransmission minimum is steeper than that in the first minimum (V₅₀ /V₁₀(1st minimum)>V₅₀ /V₁₀ (2nd minimum)). Preferably those pairs of valuesof the parameters K₃ /K₁ and Δε/ε.sub.⊥ are chosen which are below thedashed area in FIG. 1. The dashed area in FIG. 1 represent the boundarywhere the steepness of the electro-optical characteristic curve in thefirst minimum is equal to that in the second minimum. When K₃ /K₁ and/orΔε/ε.sub.⊥ are smaller, the steepness in the first minimum is betterthan that in the second minimum. Electro-optical display elements havinga steeper electro-optical characteristic curve in the first transmissionminimum than that in the second minimum are preferred.

Particularly preferred parameter combinations are K₃ /K₁ ≦0.4 andΔε/ε.sub.⊥ ≦0.3, and also K₃ /K₁ ≦0.8 and Δε/ε.sub.⊥ ≦0.05. Preferably,the smallest possible K₃ /K₁ ratio is chosen, for example 0.8 to 0.2, inparticular 0.7 to 0.3. The smaller the chosen value of K₃ /K₁, thegreater can Δε/ε.sub.⊥ be for display elements according to theinvention and the more favorable is the threshold voltage. In order toobtain a favorable threshold voltage, Δε/Δε.sub.⊥ is thus chosen to bepreferably ≧0.05, in particular ≧0.1.

The parameters used to define the invention are fully known to skilledworkers. The contemplated definitions are to be found in manypublications, e.g. W. H. De Jeu, Physical Properties of Liquid CrystalMaterials, Gordon and Breach, 1980 and the brochure "Liquid Crystals,Measurement of the Physical Properties" of E. Merck, whose disclosuresare incorporated by reference herein.

The thickness of a liquid crystal layer of the display element accordingto the invention and/or the optical anisotropy Δn of the liquid crystalmaterial are chosen such that ##EQU3## the value of 550 nm, normallyused for the maximum sensitivity of the human eye, being taken for thewavelength λ. The layer thicknesses of the display elements according tothe invention can, for example, be within the range from 2 to 10 μm.Preferably, the smallest possible layer thicknesses are chosen, thelower limit of d being determined by the quality standards required formass production. A particularly preferred range of d is thus 3 to 8 μm,in particular 4 to 7 μm.

Liquid-crystalline dielectrics, the material parameters of which arewithin the ranges according to the invention, can be prepared fromconventional liquid-crystalline base materials. Numerous such materialsare known from the literature. Advantageously, the dielectrics used forthe display elements according to the invention contain at least 0.5,preferably at least 30, in particular at least 95% by weight of amixture of at least 2, preferably 3 to 15, in particular 4 to 10liquid-crystalline compounds which each contain at least one structuralelement from the series I to VIII, it being possible for the structuralelements I to IV to be unsubstituted or also to be substituted, forexample, by F, Cl, CH₃ and/or CN or to be present in the form of thecorresponding N-oxides: ##STR1##

Preferably, these liquid crystal compounds additionally contain a1,4-phenylene group.

Such dielectrics can additionally contain dyes and/or doping substancesin the usual quantities, unless the liquid crystal parameters are thustaken out of the ranges according to the invention.

Mixing of the liquid crystalline components to achieve the parametersdiscussed above can be routinely accomplished by the usual preliminaryorientation experiments taking into account the usual considerations.Some of the many publications, whose disclosures are incorporated byreference herein, disclosing details of suitable materials includeGerman Offenlegungsschrift 2,257,588; 2,306,738; 2,017,727; 2,321,632;European Published Application 0,126,883; U.S. Pat. Nos. 3,997,536;4,062,798; 4,462,923; 4,389,329; 4,364,838; 4,066,570; 4,452,718;4,419,262; 4,510,069; Japanese Published Applications 144,770/84;144,771/84; 144,772/84 43,961/83; D. Demus et al., Flussige Kristalle inTabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1974 andD. Demus et al., Flussige Kristalle in Tabellen II, VEB Deutscher Verlagfur Grundstoffindustrie, Leipzig 1984.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. In the followingexamples, all temperatures are set forth uncorrected in degrees Celsius;unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

A TN cell with a liquid-crystalline phase consisting of

15% of 2-p-methoxyphenyl-5-hexylpyrimidine

15% of 2-p-pentoxyphenyl-5-hexylpyrimidine

15% of 2-p-heptoxyphenyl-5-hexylpyrimidine

13% of 2-p-nonoxyphenyl-5-hexylpyrimidine

15% of 2-p-methoxyphenyl-5-heptylpyrimidine

15% of 2-p-heptoxyphenyl-5-heptylpyrimidine

12% of 2-p-nonoxyphenyl-5-heptylpyrimidine (clear point 53°) shows adielectric anisotropy Δε of +0.9, and ε.sub.∥ of 4.0, and ε.sub.⊥ of3.1, a Δε/ε.sub.⊥ of 0.29, an optical anisotropy Δn of 0.168 and a K₃/K₁ of 0.40. The angle of incidence α.sub.ο on the glass surface is0.50. When operated in the first minimum (that is to say d.Δn=0.48),such a TN cell shows a steepness of the electro-optical characteristiccurve of 1.11 (V₅₀ /V₁₀), whereas the steepness is poorer in the case ofoperation in the second minimum (V₅₀ /V₁₀ =1.12).

EXAMPLE 2

A TN cell with a liquid-crystalline phase consisting of

14% of 2-p-methoxyphenyl-5-hexylpyrimidine

14% of 2-p-pentoxyphenyl-5-hexylpyrimidine

14% of 2-p-heptoxyphenyl-5-hexylpyrimidine

13% of 2-p-nonoxyphenyl-5-hexylpyrimidine

14% of 2-p-methoxyphenyl-5-heptylpyrimidine

14% of 2-p-heptoxyphenyl-5-heptylpyrimidine

12% of 2-p-nonoxyphenyl-5-heptylpyrimidine and

5% of r-1-cyano-cis-4-(trans-4-butylcyclohexyl)-1-heptylcyclohexane(clear point 55°) shows a dielectric anisotropy Δε of +0.5, a Δε/ε.sub.⊥of 0.14 and a K₃ /K₁ of 0.42. When operated in the first minimum, thiscell has a steepness V₅₀ /V₁₀ of 1.08, whereas V₅₀ /V₁₀ is 1.095 in thesecond minimum. The steepness of the characteristic curve is thereforesubstantially better in the first minimum.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. In an electro-optical display element based on aliquid-crystalline phase comprising at least two compounds, theimprovement wherein the electro-optical characteristic curve of thephase in the first Gooch and Tarry transmission minimum is at least assteep as that in the second such transmission minimum.
 2. Anelectro-optical display element of claim 1, based on a twisted nematiccell.
 3. An electro-optical display element of claim 2, wherein theratio of the parameter K₃ of said phase to the parameter K₁ of saidphase is ≦0.8.
 4. An electro-optical display element of claim 3, whereinthe ratio of the parameter Δε of said phase to the parameter Δ.sub.⊥ ofsaid phase is ≦0.3.
 5. An electro-optical display element of claim 2,wherein the ratio of the parameter Δε of said phase to the parameterε.sub.⊥ of said phase is ≦0.3.
 6. An electro-optical display element ofclaim 5, wherein the ratio of the parameter K₃ of said phase to theparameter K₁ of said phase is ≦0.8.
 7. An electro-optical displayelement of claim 5, wherein Δε/ε.sup.⊥ is ≧0.1.
 8. An electro-opticaldisplay element of claim 5, wherein the ratio of the parameter K₃ ofsaid phase to the parameter K₁ of said phase is ≦0.7-0.3.
 9. Anelectro-optical display element of claim 2, wherein the ratio of theparameter K₃ of said phase to the parameter K₁ of said phase is 0.8-0.2.10. An electro-optical display element of claim 2, wherein the ratio ofthe parameter K₃ of said phase to the parameter K₁ of said phase is0.7-0.3.
 11. An electro-optical display element of claim 10, wherein theratio of the parameter Δε of said phase to the parameter ε.sub.⊥ of saidphase is ≦0.1.
 12. An electro-optical display element of claim 2,wherein the ratio of the parameter K₃ of said phase to the parameter K₁of said phase is minimized and the ratio of the parameter Δε of saidphase to the parameter ε.sub.⊥ of said phase is maximized while thecharacteristic curve in the first transmission minimum is at least assteep as that in the second transmission minimum.
 13. An electro-opticaldisplay element of claim 12, wherein the steepness of the characteristiccurve in the first minimum is greater than that in the second minimum.14. An electro-optical display element of claim 2, wherein the steepnessof the characteristic curve in the first minimum is greater than that inthe second minimum.
 15. In a liquid crystal phase comprising at leasttwo compounds, the improvement wherein the electro-opticalcharacteristic curve of the phase in the first Gooch and Tarrytransmission minimum is at least as steep as that in the second suchtransmission minimum.
 16. A phase of claim 15, wherein the ratio of theparameter K₃ of said phase to the parameter K₁ of said phase is ≦0.4 andthe ratio of the parameter Δε of said phase to the parameter ε.sub.⊥ ofsaid phase is ≦0.3.
 17. A phase of claim 15, wherein the ratio of theparameter K₃ of said phase to the parameter K₁ of said phase is ≦0.8 andthe ratio of the parameter Δε of said phase to the parameter ε.sub.⊥ ofsaid phase is ≦0.5.
 18. A method of simultaneously minimizing thedependence of contrast on the angle of observation of an electro-opticaldisplay element based on a liquid crystal phase and of maximizing themultiplexing capacity of said element by maximizing the steepness of thecharacteristic curve of said element, comprising operating the elementin the first Gooch and Tarry transmission minimum, and selecting a phasehaving a K₃ /K₁ ratio and a Δε/ε.sub.⊥ ratio consistent with theattainment of a characteristic curve in the first transmission minimumwhich is steeper than that in the second such transmission minimum.